1. Field of the Invention
The present invention relates generally to micro-electromechanical systems (MEMS) and, in particular, to a method of encapsulating one or more MEMS devices formed on a substrate.
2. Discussion of the Background Art
Switching devices are used in electronic applications to connect and disconnect electrical signal paths. In radio frequency (RF) and microwave applications, it is desirable to form switches using micro-electromechanical system (MEMS) technology because such switches exhibit low insertion loss and high isolation capability compared to transistor switches especially at frequencies in the GHz and above. A typical MEMS switch, such as described in U.S. Pat. No. 5,619,061 to Goldsmith et al., includes an electrode in the form of a horizontal beam element with at least one end clamped, hinged or anchored to a post, spacer, via or other type of stationary vertical structure. Another electrode is disposed in opposed relation to the beam element so that, when an appropriate voltage is applied between the two electrodes, the beam element flexes in the direction of the opposed electrode, thereby closing the circuit. When the voltage is removed, the natural resiliency of the beam element returns it to its normally horizontal, open state.
A disadvantage of MEMS devices in general is their sensitivity to environmental conditions. More specifically, since MEMS devices operate through the mechanical movement of a moving member, the gap between the moving member and the opposed electrode must be clear of any particulate matter that may inhibit movement of the member. The components of a MEMS device must also be free of stiction-causing films that can develop when the device is exposed to certain environments. Stiction is an adhesive or electrostatic attraction between electrodes that can have a negative impact on the switching speed and response of the device. When the voltage potential is removed, the electrodes should separate instantaneously. Any residual, unwanted attraction between electrodes will increase the time for separation, thereby decreasing switching speed. In extreme cases, stiction may bind a MEMS switch in the closed position, thus rendering it inoperable.
One way to avoid environmental effects is to incorporate a MEMS device with other components as part of a system and to enclose the entire system in a hermetically sealed package. A disadvantage of this approach is that it dramatically increases the size and cost of the system and, in addition, tends to limit the position of the MEMS device to an exposed surface of the system. The increase in size also necessitates long vias to connect the MEMS device with radiators and other components leading to RF losses and increased noise, particularly at high frequencies (i.e., at or above 10 GHz).
Another way to avoid environmental effects is to individually enclose one or more MEMS devices in a hermetically sealed package prior to incorporating the devices into a system with other components. Ceramic packages with metal seals have been proposed for this purpose. A problem with this approach is that the mating process requires significant handling of the MEMS devices in an unprotected condition. If a number of MEMS devices are formed in a batch on a single wafer, it is also necessary to saw the wafer with the MEMS devices in an unprotected condition thereby increasing the risk of damage to the devices. It is also difficult to achieve and maintain a hermetic seal because of the variety of materials used.
Yet another approach is to form a number of MEMS devices on a first wafer and an equal number of cavities in a second wafer and to use wafer-bonding technology to join the wafers together such that the MEMS devices are each disposed within a cavity in the opposed wafer. A disadvantage of this approach is that it requires precise alignment of the wafers when bonding and complicates subsequent sawing operations. This approach also leads to a significant increase in the overall size of each MEMS device because of the need to use a relatively thick wafer for creation of the cavities.
Therefore, a need exists for an hermetically sealed MEMS device that is small, inexpensive, easy to produce, and reliable.